The present invention relates to an apparatus and method for noninvasively providing a determination of a patient""s physiological parameter and other clinically important parameters.
Blood pressure is the force within the arterial system of an individual that ensures the flow of blood and delivery of oxygen and nutrients to the tissue. Prolonged reduction or loss of pressure severely limits the amount of tissue perfusion and could therefore result in damage to or even death of the tissue. Although some tissues can tolerate hypoperfusion for long periods of time, the brain, heart and kidneys are very sensitive to a reduction in blood flow. Thus, during and after surgery, blood pressure is a frequently monitored vital sign. Blood pressure is affected, during and after surgery, by the type of surgery and physiological factors such as the body""s reaction to the surgery. Moreover, blood pressure is manipulated and controlled, during and after surgery, using various medications. Often, these physiological factors and the given medications can result in a situation of rapidly changing blood pressure requiring immediate blood pressure measurement, and corrective action.
Because of changes in the patient""s blood pressure, constant monitoring is important. The traditional method of measuring blood pressure is with a stethoscope, occlusive cuff and pressure manometer. However, this technique is slow, subjective in nature, requires the intervention of a skilled clinician and does not provide timely readings frequently required in critical situations.
For these reasons, two methods of measuring blood pressure have been developed: noninvasive, intermittent methods that use an automated cuff device such as an oscillometric cuff; and invasive, continuous (beat-to-beat) measurements that use a catheter.
The oscillometric cuff method typically requires 15 to 45 seconds to obtain a measurement, and should allow sufficient time for venous recovery. Thus, at best there is typically xc2xd to 1 minute between updated pressure measurements. This is an inordinately long amount of time to wait for an updated pressure reading when fast acting medications are administered. Also, too frequent cuff inflations over extended periods may result in ecchymosis and/or nerve damage in the area underlying the cuff. The invasive method has inherent disadvantages including risk of embolization, infection, bleeding and vessel wall damage.
To address the need for continuous, noninvasive blood pressure measurement, several systems were developed. One approach relies on blood pressure values in a patient""s finger as indicative of the patient""s central blood pressure, as in the cases of Penaz, U.S. Pat. No. 4,869,261 and H. Shimazu, Vibration Techniques for Indirect Measurement of Diastolic Arterial Pressure in Human Fingers, Med. and Biol. Eng. and Comp., vol. 27, no. 2, p. 130 (March 1989). Another system uses two cuffs, one on each arm, to determine calibration readings and continuous readings respectively. Another system transforms a time sampled blood pressure waveform into the frequency domain and determines blood pressure based on deviations of the fundamental frequency. Kaspari, et al. U.S. patent application Ser. No. 08/177,448, filed Jan. 5, 1994 provides examples of these systems. An additional class of devices, represented by L. Djordjevich et al. WO 90/00029 (PCT Application), uses electrical conductance to determine blood pressure.
A related area of interest was explored by perturbing the body tissue of patients. One class of experiments causes perturbations by inducing kinetic energy into the patient, specifically, by oscillating a blood vessel. In the work of Seale, U.S. Pat. No. 4,646,754, an attempt is described to measure blood pressure by sensing the input impedance of a blood vessel exposed to a low frequency vibration. In work by H. Hsu, U.S. Pat. No. 5,148,807, vibrations are used in a non-contact optical tonometer. Several experiments measured the velocity of excited perturbations in the blood and demonstrated a correlation between perturbation velocity and blood pressure. Such a correlation has also been demonstrated between pressure and the velocity of the natural pulse wave. However, while these studies discuss the relationship between velocity and pressure they do not propose a practical method of measuring induced perturbations to determine blood pressure. Examples of such studies are M. Landowne, Characteristics of Impact and Pulse Wave Propagation in Brachial and Radial Arteries, J. Appl. Physiol., vol. 12, p. 91 (1958); J. Pruett, Measurement of Pulse-Wave Velocity Using a Beat-Sampling Technique, Annals of Biomedical Engineering, vol. 16, p. 341 (1988); and M. Anliker, Dispersion and Attenuation of Small Artificial Pressure Waves in the Canine Aorta, Circulation Research, vol. 23, p.539 (October 1968).
Known techniques for measuring propagation of pressure perturbations in arteries include Tolles, U.S. Pat. No. 3,095,872 and Salisbury, U.S. Pat. No. 3,090,377. Tolles employs two sensors to detect a perturbation waveform and generate two sensor signals. The two sensor signals are compared in a phase detector. The phase difference of the sensor signals is displayed giving a signal that is capable of detecting changes in blood pressure, but which does not provide a calibrated blood pressure output. Salisbury similarly employs a sensor to detect a perturbation waveform and generate a single sensor signal. The sensor signal is compared against a reference signal. Based on the phase difference of the sensor signal, a universal formula is employed to determine the patient""s blood pressure. Since it has been shown, for example by Landowne, that the relationship between pressure and signal propagation varies considerably from patient to patient, Salisbury""s technique, based on a single formula, is not generally applicable.
The present invention describes an apparatus and method for measuring the induced perturbation of a patient""s body tissue to determine the patient""s blood pressure and other clinically important parameters.
An object of the present invention is to continuously determine a patient""s blood pressure via a noninvasive sensor attached to the patient.
A related object is to induce a perturbation into a patient""s blood or blood vessel and to noninvasively measure the perturbation to determine the patient""s blood pressure.
A related object is to filter the noninvasive sensor signal into components including a natural component, an induced component and a noise component, and to determine the patient""s blood pressure from the induced component.
A further related object is to determine a relationship between a property of an induced perturbation and a property of a physiological parameter.
A monitor for determining a patient""s physiological parameter includes a calibration device configured to provide a calibration signal representative of the patient""s physiological parameter. An exciter is positioned over a blood vessel of the patient for inducing a transmitted exciter waveform into the patient. A noninvasive sensor is positioned over the blood vessel, where the noninvasive sensor is configured to sense a hemoparameter and to generate a noninvasive sensor signal representative of the hemoparameter containing a component of a received exciter waveform. In this context, a hemoparameter is defined as any physiological parameter related to vessel blood such as pressure, flow, volume, velocity, blood vessel wall motion, blood vessel wall position and other related parameters. A processor is configured to determine a relationship between a property of the received exciter waveform and a property of the physiological parameter. The processor is connected to receive the calibration signal and the noninvasive sensor signal, and the processor is configured to process the calibration signal and the noninvasive sensor signal to determine the physiological parameter. In the preferred embodiment, the physiological parameter measured is blood pressure, however, the present invention can also be used to analyze and track other physiological parameters such as vascular wall compliance, strength of ventricular contractions, vascular resistance, fluid volume, cardiac output, myocardial contractility and other related parameters.